Understanding the computational rules of the brain requires studying neuronal activity at the network (submillimeter) level. This is the domain where emerging properties of neuronal interactions can be quantitatively measured and models testing the biologically based hypotheses can be developed. Direct investigation of the temporal dynamics of neuronal populations can only be based on simultaneous observation of many individual neurons in the intact animal. These network dynamics give rise to coherent local field potentials which may provide an insight into the mechanism of neuronal interactions, a major goal of this proposal is to develop a high temporal resolution method for imaging of neuron- generated local fields together with simultaneous recording of single neuron activity in large neuronal ensembles in the behaving animal. Specifically, we will develop procedures (a) for mapping of extracellular field activity onto the real structure of the hippocampal formation in the intact rat brain and (b) fo recording large number of isolated cells and local field in a small volume of brain tissue. The proposed goals will be achieved by using high-density silicon probes and developing automatic unit clustering algorithms. These tools will allow us to investigate the cellular-synaptic generation of hippocampal theta and gamma oscillation in various hippocampal subregions and to examine how oscillations bind and segregate neuronal assemblies in the hippocampus. The electric field-unit imaging methods developed and tested in the hippocampus can be applied for other cortical and subcortical structures.